MuseScore

MuseScore is a free cross-platform WYSIWYG music notation program, that offers a cost-effective alternative to professional programs such as Sibelius and Finale.
You can print beautifully engraved sheet music or save it as PDF or MIDI file.
Some highlights:
* WYSIWYG, notes are entered on a "virtual note sheet"
* Unlimited number of staves
* Up to four voices per staff
* Easy and fast note entry with your keyboard, mouse, or MIDI keyboard
* Integrated sequencer and FluidSynth software synthesizer
* Import and export of MusicXML and Standard MIDI Files
* Available for Windows, Mac and Linux
* Translated in 26 languages
* GNU GPL licensed
See http://www.musescore.org/